[Hormone production and abnormalities in gene expression in tumors]

In order to elucidate the mechanism responsible for ectopic hormone production in tumors, biosynthesis of ACHT and related peptides was studied in the pituitary and tumors at levels. In vitro biosynthesis of the ACTH/beta-LPH precursor directed by mRNA extracted from the pituitary and tumors showed no difference in translation products. It is highly likely, therefore, that different final products produced in the pituitary and tumors are caused by different posttranslational processing, such as proteolysis and glycosylation of translation products. The RNA blot analysis of tumors revealed mRNA identical to that of the pituitary. In certain tumors, however, there were larger or smaller mRNA hybridized with an ACTH/beta-LPH precursor probe, in addition to mRNA of normal size. Further studies with endonuclease S1 mapping have provided evidence suggesting that the larger one was probably produced by abnormal splicing of RNA precursor and that the smaller one was resulted possibly from aberrant transcription of the gene. The Southern blot analysis revealed no difference in restriction DNA fragments between the pituitary and tumors, indicating no evidence of gene rearrangement. From these studies, it is conceivable that ectopic ACTH production is resulted from abnormalities in the regulatory mechanism of gene expression. To further study the mechanism regulating the expression of the ACTH/beta-LPH precursor gene, human gene was transfected to mouse pituitary ACTH-producing adenoma cells (AtT-20) and fibroblasts (L-cell). The introduced human ACTH/beta-LPH precursor gene was expressed in AtT-20 cells and suppressed by glucocorticoids to an extent similar to the suppression of mouse gene. On the other hand, possible aberrant transcripts were observed in mouse L cells. It is likely, therefore, that there is a regulatory mechanism, probably "trans" acting, in the pituitary ACTH-producing producing cells and similar mechanism, though not identical, could be exerted in ectopic ACTH-producing tumors.